1. Field of the Invention
The present invention relates to curable sol/gel compositions for preparing transparent antistatic, impact resistant and/or abrasion resistant coatings, articles exhibiting antistatic, impact resistance and/or abrasion resistance property coated therewith, in particular optical and ophthalmic glasses for eyeglasses, and a process to prepare such articles.
2. Description of Related Art
It is well known that optical articles, which are essentially composed of insulating materials, have a tendency to get charged with static electricity, especially when they are cleaned in dry conditions by rubbing their surface with a cloth or synthetic piece, for example a polyester piece (triboelectricity). The charges which are present at the surface of said optical articles create an electrostatic field capable of attracting and fixing, as long as the charge remains on optical articles, objects lying in the vicinity thereof (a few centimeters) that have a very little weight, generally small size particles such as dusts.
In order to decrease or suppress attraction of the particles, it is necessary to decrease the intensity of the electrostatic field, i.e. to decrease the number of static charges which are present at the surface of the article. This may be carried out by imparting mobility to the charges, for instance by introducing in the optical article a layer of a material inducing a high mobility of the charges. Materials inducing the highest mobility are conductive materials. Thus, a material having a high conductivity allows for a more rapid dissipation of charges.
It is known in the art that an optical article acquires antistatic properties owing to the incorporation at the surface thereof of at least one electrically conductive layer, which is called an antistatic layer.
By “antistatic”, it is meant the property of not retaining and/or developing an appreciable electrostatic charge. An article is generally considered to have acceptable antistatic properties when it does not attract or fix dust or small particles after one of its surfaces has been rubbed with an appropriate cloth. It is capable of quickly dissipating accumulated electrostatic charges.
The ability of a glass to evacuate a static charge created by rubbing it with a cloth or any other electrostatic charge generation process (charge applied by corona discharge) can be quantified by measuring the time required for said charge to be dissipated (charge decay time). Thus, antistatic glasses have a discharge time in the order of less than 500 milliseconds, and generally around 100-200 milliseconds, while static glasses have a discharge time in the order of several tens of seconds, sometimes even several minutes. A static glass having just been rubbed can thus attract surrounding dusts as long as it requires time to get discharged.
Only a limited number of materials are known in the art for preparing electrically conductive inorganic or organic layers having high optical transparency, i.e. a transmittance in the visible light of at least 90%. Known optically transparent antistatic coatings include vacuum-deposited metal or metal oxide films, for example films based on optionally doped (semi) conductive metal oxides such as tin oxide doped with indium (ITO), tin oxide doped with antimony (ATO) or vanadium pentoxyde, or spin-coated or self-assembled conductive polymer films.
ITO is the industry standard antistatic agent to provide optically transparent electrically conductive thin coatings, but the performance of ITO suffers when it is applied to plastics. These coatings are fragile and are readily damaged during bending or other stress inducing conditions. In addition, ITO layers need to be deposited by vacuum deposition in a controlled gas atmosphere.
Conductive polymers represent the most investigated alternative to ITO coatings. They are generally formed from a liquid coating composition, but still cannot match the optical and electrical performances of ITO and sometimes suffer from environmental stability problems in specific applications.
Organic-inorganic hybrid polymer electrolytes have also been proposed to form antistatic coatings.
Thus, the article entitled “Sol-gel preparation of organic-inorganic hybrid polymer electrolytes and their electrochemical characterizations” (Myong-Hoon Lee, Sung Tae Ko, Kwang Sup Lee and Suh Bong Rhee, Mol. Cryst. Liq. Crypt. 1997 Vol. 294 pp 229-232), relates to a sol/gel coating composition that includes tetraethoxysilane (TEOS), polyethylene oxide of low molecular weight (PEO, Mw=300) and an alkoxysilane bearing a polyethylene oxide group. The latter component is used as a compatibilizing agent, in order to avoid a phase separation between the “free” PEO of low molecular weight and the polysiloxane matrix. The sol/gel coating composition of the above article contains an important amount of such free PEO. The antistatic coatings made from the above disclosed composition are very easily dissolved and removed in aqueous or organic solvents, for example water and methanol. Thus, these coatings cannot be used when a further coating is to be deposited thereon through a wet coating technique, e.g. spin or dip coating. Additionally, another problem is that the originally transparent coating becomes becoming hazy.
Japanese patent application No 2007-321014 discloses a composition for forming a polyurethane based antistatic layer which comprises a resin A which is a polymeric component having an active hydrogen group (typically a polyvinyl alcohol polymer), a resin B which is an ion conductive polymer, a supporting electrolytic salt and a polyisocyanate. The ion conductive polymer of resin B contains a hydrophilic segment, a polysiloxane segment and an active hydrogen group. The hydrophilic segments can be PEO segments. The content of the polysiloxane segments shall not exceed 30% by weight of the whole resin, otherwise the resulting antistatic film becomes slippery. This composition is specifically designed to provide a coating having a polyurethane matrix.
It is still desirable to produce new antistatic sol/gel compositions which result in antistatic coatings that do not impair the optical and mechanical properties of the coated optical article.
In particular, the new antistatic compositions shall provide antistatic coatings capable of being themselves coated through a wet coating technique with additional coatings such as an abrasion-resistant and/or scratch resistant coating, while keeping their excellent antistatic properties.
A further goal of the invention is to provide such antistatic compositions for forming antistatic and optionally abrasion and/or scratch-resistant layers that provide both good adhesion to an optical substrate and/or an additional functional coating formed thereon, and at the same time preserve the optical and mechanical properties of the obtained optical article, for example an ophthalmic lens; (said properties including high transmittance, low haze, good abrasion/scratch resistance, good impact resistance and other additional properties such as antireflection, anti-smudge, anti-fogging, etc.).
Further, soft plastic substrates such as plastic lenses are generally made of soft polymer materials and are easily scratched especially when the lenses are cleaned using a cloth and especially when there are dust deposited on the surfaces. Thus, an abrasion and/or scratch resistant coating is usually needed for plastic lenses or other soft substrates.
Accordingly, another objective is to provide an antistatic, abrasion and/or scratch-resistant composition for forming an antistatic and anti-abrasion coating.
Also, an aim of the invention is to provide antistatic compositions as above which result in antistatic coatings having a polysiloxane matrix rather than a polyurethane matrix for improved compatibility with classic abrasion resistant coatings which are mostly polysiloxane based coatings.